(1) Field of the Invention
The invention relates to a circuit for protecting a battery, and more particularly, to a circuit for controlling the charging, discharging, and protection of a rechargeable battery.
(2) Description of the Prior Art
Rechargeable batteries are used in a variety of applications of portable electronic devices. In particular, rechargeable batteries are used for portable phones. Several types of batteries are used in the art, most notably those comprising lithium ion or Li+. For optimum battery life and performance, the battery must be protected from excessive charging voltage during recharging and from over discharging while supplying the appliance.
Referring now to FIG. 1, an exemplary prior art battery protection circuit. A rechargeable battery 10 is configured such that it can source energy to a load 34 or can be recharged by a charger source 38. Two MOSFET switches N118 and N226 are used to control the flow of current into (charging) and out of (discharging) the battery. The control circuit 14 controls the ON and OFF state of the switches 18 and 26. Typically, the switches comprise discrete devices. Each switch 18 and 26 contains a parasitic bulk-to-drain diode D122 and D230. By coupling each switch in an opposite manner, each of the parasitic diodes 22 and 30 conducts current in a different direction.
During normal charging or discharging of the battery 10, both of the switches are ON. However, if the control circuit 14 detects an over charging voltage from VBATT to GND, then the control circuit turns OFF the switch N1. Since the parasitic diode D122 also blocks current flow into the battery, the battery stops charging. In the case of an over discharging condition, the control circuit 14 would detect a too low battery voltage from VBATT to GND. The other switch N226 would be turned OFF. In this case, the parasitic diode D230 blocks current flow out of the battery 10.
The prior art circuit has at least two disadvantages. First, two FET switches are required in order to obtain two-directional switching capability. Therefore, the series resistance of the battery pack is increased. Second, the use of discrete components increases the manufacturing cost and space requirements for the protection circuit.
Several prior art inventions describe battery protection circuits and devices related to bulk switching. U.S. Pat. No. 6,246,214 to Oglesbee shows a battery protection circuit having only one MOSFET switch. A voltage protection circuit is used to limit the voltage so that a single MOSFET can be used for two directional current control. U.S. Pat. No. 5,063,471 to Park discloses a battery protection circuit. U.S. Pat. No. 5,789,900 to Hasegawa et al discloses a battery protection circuit showing two MOSFETs having back-to-back diodes. U.S. Pat. No. 6,160,381 to Peterzell discloses a battery pack protection circuit. Two MOS switches are used. U.S. Pat. No. 5,081,371 to Wong describes a charge pump circuit where back gate biased MOSFETs are used. U.S. Pat. No. 5,933,046 to Ramet et al teaches an analog switch formed from a MOSFET that has a switchable bulk connection.
A principal object of the present invention is to provide an effective and very manufacturable circuit for protecting a battery.
A further object of the present invention is to provide a battery protection circuit for controlling charging and discharging of a battery.
A still further object of the present invention is to control both charging and discharging via a single FET having a switchable bulk connection.
Another still further object of the present invention is to cascade switchable bulk FET devices to increase the voltage range of the circuit.
Yet another still further object of the present invention is to charge the middle node between cascaded FET devices to improve performance.
Another still further object of the present invention is to form the switchable bulk FET devices using either NMOS or PMOS devices.
In accordance with the objects of this invention, a battery charging, discharging, and protection circuit is achieved. The circuit comprises, first, a FET switch having gate, source, drain, and bulk. The FET switch may comprise either a NMOS device or a PMOS device. The source is coupled to a load terminal, and the drain is coupled to a battery terminal. Second, a means of controlling the FET switch gate and the bulk is included. The FET switch gate voltage determines the OFF and ON state of said FET switch. The FET bulk is switchably coupled between the battery terminal and the load terminal.
Also in accordance with the objects of the present invention, a battery charging, discharging, and protection circuit is achieved. The circuit comprises, first, a first FET switch having gate, source, drain, and bulk. The drain is coupled to a battery terminal. Second, a second FET switch having gate, source, drain, and bulk is used. The drain is coupled to the first NMOS FET switch source to thereby form a mid node. The source is coupled to a load terminal. Finally, a means of controlling the first and second FET gates and the switchable bulks is used. The first and second FET switch gates voltages determine the OFF and ON states of the first and second FET switches. The first FET bulk is switchably coupled between the battery terminal and the mid node. The second FET bulk is switchably coupled between the mid node and the load terminal.